This invention relates to the field of drop marking systems of the type in which a liquid ink is forced under pressure through a nozzle which converts the liquid into droplets which can then be controlled by various means while projected toward a substrate for marking purposes. Examples of such systems include the familiar ink jet marking systems used for high speed label printing, product identification and the like, although there are other drop marking systems known in the art. One particular type of system which advantageously employs the present invention is the continuous stream, synchronous ink jet printer. Such a system typically includes an ink reservoir and a remotely located nozzle connected to the reservoir by a conduit. Ink is forced under pressure from the reservoir to the nozzle which emits a continuous stream of ink drops. The ink, which is electrically conductive, is provided with a charge as the drops leave the nozzle. The drops then pass through a deflection field which causes selected drops to be deflected so that some of the drops are deposited onto a substrate while the remaining drops are returned to the reservoir by a suitable ink return means.
In order to produce high quality marking, it is important that the ink drops pass through the deflection field at a relatively constant velocity. Thus, ink drops with similar charges but different velocities will experience unequal amounts of deflection resulting in inconsistent print quality.
The condition of constant ink drop velocity through the deflection field requires that the flow rate of liquid through the nozzle be substantially constant. Prior ink marking systems have attempted to accommodate this requirement by various means. None, however, has been entirely successful measured in terms of simplicity, cost, reliability and overall accuracy of the resulting function.
One class of prior art devices attempt to obtain constant velocity by using constant ink delivery pressure in conjunction with a system of indirect viscosity control. These devices, manufactured by the assignee of the present invention and disclosed in U.S. Pat. Nos. 3,930,258 and 4,121,222, employ constant volume ink reservoirs. The amount of ink solvent evaporative loss is measured either by weighing the reservoir or by measuring the volume change. Ink loss due to marking is replenished by using a plurality of make up ink formulations or by using a drop counter. The accuracy of the latter approach is limited by the fact that the volume of ink lost is calculated, not measured and thus the volume of replacement ink required is only an approximation of the correct amount.
Another prior art system, disclosed in U.S. Pat. No. 4,337,468, counts printed drops as well as measuring the amount of ink returned to the system. This information is used to calculate the amount of evaporative loss and additional solvent is added in response thereto. This technique is open loop (no feedback control) and does not permit the degree of accuracy desired to insure essentially constant velocity through the deflection field of the ink jet device.
Other efforts to deal with these problems are known in the prior art. One such system employs a specific gravity detector which signals when it is necessary to add solvent to the ink supply. This system overcomes the drawbacks of drop counting but is unsuitable for use in systems where the printer must accommodate many different types of inks, each with its own specific gravity parameters. Further, in general, these devices do not provide good determinations as to the viscosity of the ink and as a result, additional viscosity control is required as by use of a heating device in the ink supply system, such heating system being referenced against ambient temperature rather than any flow property of the ink.
Another commercial system which tries to deal with the problem of changing ink viscosity is manufactured by the IBM Corporation. In this device the ink pressure is resposive to signals from a deflection detector. The deflection detector is located in the electric field through which the drops pass. The detector signals the pump to increase or decrease pressure, as necessary, to maintain drop velocity at an appropriate value. This system provides feedback control of drop velocity. The technique, however, is not entirely satisfactory because of the complexity and cost of the components and the need for a fragile deflection detector at the remote print head location.
Other available ink jet systems employ viscometer for adjusting the viscosity of the ink. Such systems are unduly complex and expensive and the results of such techniques still do not provide direct feedback control with respect to the drop velocity through the charge field. Control is indirect based on the viscosity of the ink.
The present invention, by sensing the flow of the ink from the reservoir and generating ink flow rate data, monitors the velocity of the drops of ink in the charge field and adjusts the ink parameters to maintain a desired flow rate which insures a substantially constant drop velocity. In effect, the present invention provides direct control over the velocity of the ink drops and does so by use of low cost components arranged in a simple manner.
It is an object of the present invention to incorporate direct feedback control into an ink drop velocity control system which is simple, reliable and low in cost.
Another object of the invention is to provide a velocity control system for an ink jet printer which maintains the velocity of ink through a deflection field substantially constant thereby insuring accurate location of drops on the substrate to be marked.
A further object of the invention is to provide an electronic control system for an ink jet printer to permit accurate control of the addition of solvent to the system.
Another object of the invention is to provide a flow control means for an ink jet system which is located entirely separate from the print head nozzle and yet maintains a substantially constant flow rate through the nozzle.
Other objects and advantages of the invention will be apparent from the remaining portion of the description.